Shallow roots, and ideas for covering them

We have some mature cottonless cottonwood trees that are beautiful, providing great shade but their roots are on the top of the lawn. So I have these huge woody roots all over my yard, not allowing me to mow. This started a couple years ago. Our three trees get their water from the sprinkler system that waters the grass. Is there anything we can do to stop this? Would I endanger the trees if I take a saw to the above-ground roots that are taking over the yard?

— Pat

You’re dealing with a problem that a lot of people struggle with. Shallow roots can be an aggravating thing to deal with as you bump and jump over them with the lawn mower.

But it’s a BAD idea to cut off the offending roots. You’d be depriving the plant of a potentially significant portion of the root system as well as making open wounds that could lead to secondary disease or insect problems as well as potentially destabilizing the tree.

There are three common reasons why plants develop shallow surface roots.

First, sometimes it’s just the nature of the plant. Some trees tend to have shallower roots than others.

Cottonwoods are a member of this group along with willow, birch, maple and poplar. These plants are usually riparian in nature, that is, they naturally grow along streams and rivers and they have developed shallow roots to cope with the often waterlogged soil conditions they contend with in nature. You can help them develop a deeper root system (more about that below), but their natural tendency is to put out shallow roots.

The second reason for shallow roots is the nature of our soils. We tend to sit on heavy clays that not only don’t drain well, but are also not well-aerated. Roots need both water and oxygen and will grow in response to whatever need they have.

Since our heavy clays don’t naturally have the pore space for oxygen to get down deep, the roots of plants will grow in more shallow levels because that’s the only place where they can breathe. This is why we’re such big advocates of good soil preparation before planting. Mixing in good amounts of decomposed organic matter helps to loosen and open up that tight clay so air can penetrate, resulting in a deeper root system for your tree.

The last reason involves the care the plant receives. This goes back to what I was talking about earlier. We can encourage shallow roots by how we water plants.

Frequent, shallow irrigations (which are what we usually find in lawn situations) will result in shallow roots. This is because the roots of plants aren’t dumb! They’ll grow where conditions are favorable. If the water is only penetrating 6 or 8 inches, that’s where the roots will be.

I frequently see shallow moisture in lawn situations and most of the time, people are unaware of it. That’s because moisture that deep is fine for the grass, but trees really benefit from a deeper soaking. It’s a good idea to occasionally (once every month or two) give the tree a deep soaking.

The frequency of the watering also plays a part. If the lawn is watered often, there’s little oxygen down deep so the roots will come up into the shallow layers to breathe. Try to water your lawn deeply, but allow the soil to dry a bit before soaking it again.

I suppose in a perfect world, trees wouldn’t be planted in the lawn since their water needs are so different, but obviously, that’s not the case, and besides, that doesn’t help you right now.

About the best I can recommend is a Band-Aid approach of gradually filtering in some loose, sandy soil over the grass between the surface roots.

If you imagine the roots and soil in cross-section, you have two “hills” where the roots are with a lower “valley” of lawn between. Place a shallow layer of soil in the valley to help even things up a bit.

You don’t want to go too deep at one time, probably no more than 1 inch of soil per year. If you go too deep at once, you’ll have to reseed the lawn (which may happen anyway to a small extent doing it gradually), but more importantly, you could potentially hurt the tree. Go slowly and allow the tree to become accustomed to the new soil level.

It may take you two, three or even four years to smooth things out, but you’ll eventually get rid of the obstacle course in your lawn.

That’s the good news. The bad news is that all this is temporary. As time goes on and those surface roots continue to grow, they’ll get thicker and eventually they’ll erupt again from the soil.

It’s not that they’ve moved up in the soil, they’re still at the same depth, but as they grow in girth, they show up again. When this happens, just gradually, add a bit more soil like you did before.

Dennis Hill is the nursery manager at Bookcliff Gardens, bookcliffgardens.com. Send questions to Bookcliff Gardens, 755 26 Road, Grand Junction 81506; or email .(JavaScript must be enabled to view this email address).


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